A method by which a seed layer is provided by electroless copper plating and a copper film is formed by copper electroplating is known as a method for forming a copper film of an ULSI fine copper wiring (damascene copper wiring).
When electroless copper plating is performed on a mirror finished surface such as a semiconductor wafer by a conventional method, it is difficult to obtain sufficient adhesion for the deposited plated film. Furthermore, the plating reactivity is low and uniform plating is difficult to perform over the entire surface of the substrate. For example, the problems arising in a case where a copper seed layer is formed by a conventional electroless plating method on a barrier metal layer, such as tantalum nitride, are that a uniform plated layer is difficult to form and adhesive strength of the plated layer is insufficient.
The inventors of the present invention have previously discovered that the deposition rate in plating can be reduced, crystal grains can be greatly refined, and a uniform thin film with a thickness of equal to or less than 15 nm can be formed on a mirror finished surface such as a wafer by adding a water-soluble nitrogen-containing polymer with a small weight-average molecular weight (Mw) as an additive to an electroless copper plating solution, and adhering a catalytic metal to a substrate that is a material to be plated prior to immersion into the plating solution or immersing the substrate into the plating solution after a catalytic metal film has been deposited in advance on the outermost surface, so that the polymer is adsorbed via nitrogen atoms on the catalytic metal (Patent Document 1). Furthermore, in the examples of the aforementioned invention, the inventors have demonstrated that the deposition rate in plating can be reduced, crystal grains can be greatly refined, and a uniform thin film with a thickness of equal to or less than 6 nm can be formed on a mirror finished surface such as a wafer by immersing the substrate into the plating solution after the catalytic metal film has been deposited in advance on the outermost surface, and causing the adsorption of the polymer via nitrogen atoms on the catalytic metal.
Where a copper seed layer is provided by electroless plating after the catalytic metal film has been deposited in the above-descried method, that is, in the method for forming the damascene copper wiring, a barrier layer serving to prevent the diffusion of copper has to be formed in advance separately from the catalytic metal layer. Therefore, a layer consisting of two layers, that is, the barrier layer and the catalytic metal layer, is formed before the copper seed layer is formed. The resultant problem is that with the ultrafine wiring, which does not allow for a large film thickness, applications to actual processes are difficult.
The inventors have discovered that the difficulty of forming two layers prior to the formation of the copper seed layer can be resolved by forming a single layer composed of a specific thin alloy film having both the barrier capability and the catalyst capability and using a combination of substitution plating and reduction plating during electroless plating because such an approach makes it possible to form a thin and uniform copper seed layer on the produced single layer. Patent applications that cover this technology have already been filed (Patent Document 2 and Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-223100.    Patent Document 2: International Patent Publication No. 2009/016979, Pamphlet.    Patent Document 3: International Patent Publication No. 2009/016980, Pamphlet.